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Atex 100 a
implications for dry-running mechanical seals?
The new explosion protection directive presents a challenge to all
machine and component manufacturers who make products that are used in
explosion risk zones. Non-electrical ignition sources have now been added
to the scope of the directive. This includes dry-running mechanical seals
that can generate very high temperatures. When suitable measures are
taken, even single mechanical seals can be operated without a quench at an
acceptable level of safety.
On March 23rd 1994, directive 94/9/EG, better known as Atex 100a (an
abbreviation for the French term atmosphères explosibles), was approved by
the European Council, and it became German law on 19/12/1996. The primary
purpose of this directive was to standardize national directives relating
to explosion protection in the member states. Atex 100a must be
implemented by July 1st, 2003. The directive applies not only to ignition
sources relating to electrical systems, devices and components, but also
to all device components (including non-electrical components), which can
act as an ignition source due to elevated surface temperatures. Atex 100a
thus applies to pump components such as bearings, impellers and mechanical
seals. Seals require a sufficient film of lubricant. In the absence of
lubricant, the seals can very quickly reach very high temperatures.
Dry-running mechanical seals
To perform a quantitative study on the effect of insufficient lubrication
on mechanical seals, a dry-running condition was intentionally created on
a test stand, and the temperature at the stationary seat was measured. A
gate valve was closed on the suction side of the pump. The result of the
measurements is shown in Fig. 1. As you can see, within a few seconds
dry-running increased the temperature at the seat from 20° C to over 120°
C. To avoid permanent damage to the elastomer material in the seals, the
gate valve on the suction side was opened again when the temperature
reached about 120° C. Similar tests have shown that dry-running can cause
temperatures at the mechanical seal to rise to over 600° C. Therefore,
these seals must be regarded as potential ignition sources in explosive
atmospheres. The results show that dry-running must be avoided at the
mechanical seal under all circumstances.
Mechanical seals
and Atex 100a
For zone 1, Atex 100a only requires certification of devices. Pumps or
agitators are devices as defined by Atex 100a. Mechanical seals, on the
other hand, are components as defined in Article 4, paragraph 2, which
cannot be certified on their own for zone 1. As a result, it is initially
sufficient if the manufacturer indicates in the operating instructions
that dry-running of the seal must not occur during operation of the device
(for example pumps). If the operator of the device cannot ensure this,
steps must be taken to preclude the generation of excessive surface
temperatures. Different steps must be taken depending on the type and
configuration of the seal. The most common multiple configurations used in
practical applications are tandem and back-to-back configurations.
Tandem configuration
In this configuration, two mechanical seals are arranged one behind the
other in “series”. Fig. 2 a) shows a diagrammatic representation of this
configuration. The seal which faces towards the product actually seals the
product. The second seal on the outboard side only performs a sealing
function if the primary seal fails. There is usually a suitable fluid
between the two mechanical seals, which can be cooled if necessary. It is
clear from the diagram that during dry running of the inboard mechanical
seal, the temperature rise is limited by contact with the fluid quench. In
the tandem configuration, it is therefore sufficient to monitor the level
in the supply reservoir and if necessary to monitor the temperature of the
buffer fluid as well. This configuration is shown in Fig. 2 b). The two
seal faces point in opposite directions. In this configuration, the buffer
fluid is normally pressurized at about 2-3 bar above the product pressure
level. The buffer fluid lubricates both the inboard and the outboard
mechanical seals in the back-to-back configuration. As a result, absence
of the product does not lead to dry-running in the back-to-back
configuration. Monitoring the pressure in the buffer fluid is thus
sufficient to prevent dry running.
Single mechanical seals
By far the most commonly used mechanical seals are so-called single
mechanical seals. Again, they can be divided into two configurations,
namely single seals with quench and without quench.
Single mechanical
seal with quench
This configuration functions in a similar manner to the tandem
configuration except that instead of a second outboard mechanical seal, a
lip seal or throttle is used. Fig. 3 a) shows a diagrammatic
representation of this configuration. In this configuration too,
dry-running does not immediately lead to very high surface temperatures,
because the quench fluid is present at the mechanical seal. As with the
tandem configuration, it is sufficient to monitor the level in the supply
reservoir and possibly the temperature of the quench fluid as well.
Single seal without quench
Fig. 3 b) shows a diagrammatic representation of this configuration. A
single mechanical seal is used for sealing the product. Because no fluid
is present at the outboard side of the seal, dry-running and potentially
even a partial lack of lubrication by the product can lead to elevated
temperatures at the seat. If the operator of the pump or agitator cannot
ensure that product is present at all times at the seat, additional
monitoring measures must be taken. Direct temperature monitoring at the
seat is one possibility, for which purpose a sensor can be mounted in the
seat. A suitable temperature controller can read out data from the sensor
and shut off the device when a pre-set temperature threshold is exceeded.
Standard temperature sensors can be used, which must however have small
dimensions. These sensors are normally electrical components (resistance
thermometers, thermal elements, etc.). However, both the sensor and the
temperature controller must conform to Atex 100a requirements. This means
in particular that the sensor must not represent an ignition source as
defined in Atex 100a.
Requirements that apply to software used to monitor safety-related
quantities are a special aspect of Atex 100a. The directive stipulates
that no software may be used during measurement of safety-related
temperatures (seat temperature, see above). It must however be emphasized
that this requirement is still under discussion; it could be that the use
of software may be allowed if particular attention was paid to safe,
stable software operation.
Summary
In addition to existing explosion protection requirements, Atex 100a also
includes non-electrical ignition sources. Mechanical seals must be given
particular consideration, because in the absence of sufficient lubrication
they can reach very high temperatures. In a number of practice-proven seal
configurations it is sufficient to monitor the buffer system. It is of
course necessary to ensure that the buffer system and all monitoring
equipment also conform to Atex 100a. Special measures only need to be
taken if single mechanical seals without quench are used. One option is to
directly measure the temperature at the stationary seat of the mechanical
seal.
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